1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and particularly to the field of marine seismic data processing. More particularly, the invention relates to multiple attenuation in dual sensor towed marine seismic streamers.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downwardly into the subsurface of the earth. In a marine seismic survey, the seismic signal will first travel downwardly through a body of water overlying the subsurface of the earth.
Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflections are detected by seismic sensors (also called receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data is recorded and processed to yield information relating to the geologic structure and properties of the subterranean formations and their potential hydrocarbon content.
Appropriate energy sources for seismic surveys may include explosives or vibrators on land and air guns or marine vibrators in water. Appropriate types of seismic sensors may include particle motion sensors in land surveys and water pressure sensors in marine surveys. Particle motion sensors are typically particle velocity sensors, but particle displacement, particle acceleration sensors, or pressure gradient sensors may be used instead of particle velocity sensors. Particle velocity sensors are commonly known in the art as geophones and water pressure sensors are commonly known in the art as hydrophones. Both seismic sources and seismic sensors may be deployed by themselves or, more commonly, in arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers have been proposed that contain water particle velocity sensors such as geophones or particle acceleration sensors such as accelerometers, in addition to hydrophones. The pressure sensors and particle motion sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable.
Recorded seismic data contains signal in terms of the useful primary reflections (“primaries”) as well as noise, such as multiple reflections (“multiples”). Primaries are single reflections from subsurface seismic reflectors of interest, while multiples are multiple reflections from any combination of reflectors. Multiples are especially strong relative to primaries in marine seismic surveys, because the water-earth and, particularly, the air-water interfaces are strong seismic reflectors due to their high acoustic impedance contrasts. Surface related multiple reflections, in particular, are those multiples that have at least one downward reflection at the free surface (water-air contact). The number of downward reflections at the surface defines the order of the surface related multiples. Under this definition, primaries are just zero order surface related multiples. Thus, a method is desired that removes first and higher order surface related multiples.
Many of the conventional methods apply seismic processing to pressure sensors only. However, the pressure sensor data has spectral notches caused by the water surface reflections, commonly referred to as sea surface ghosts. These spectral notches are often in the seismic acquisition frequency band. Thus, the usable portion of the pressure sensor data is frequency band limited away from the spectral notches and cannot cover the entire seismic acquisition frequency band. This limitation can be avoided by using both pressure sensors and particle motion sensors in a “dual sensor” streamer.
L. Amundsen and A. Reitan, in their article “Decomposition of multicomponent sea-floor data into upgoing and downgoing P- and S-waves”, Geophysics, Vol. 60, No. 2, March-April, 1995, p. 563-572, describe a method for deghosting dual sensor cable data in the water layer and on the sea floor. Amundsen and Reitan construct a decomposition filter to apply to pressure recorded by hydrophones just above the sea floor and the radial and vertical components of the particle velocity recorded by geophones just below the sea floor. The decomposition filter separates the data into upgoing and downgoing P- and S-waves, yielding the deghosted wavefield in the up-going components. The decomposition filter coefficients depend upon the P- and S-wave velocities and the density at the sea floor.
Borresen, C. N., in U.S. Patent Publication No. US 2006/0050611 A1, entitled “System for Attenuation of Water Bottom Multiples in Seismic Data Recorded by Pressure Sensors and Particle Motion Sensors”, assigned to an affiliated company of the assignee of the present invention, describes a method for attenuation of water bottom multiples in marine seismic data. The method includes calculating up-going and down-going wavefield components from pressure sensor and particle motion sensor signals, extrapolating the wavefields to the water bottom, and utilizing the extrapolated wavefields and a water bottom reflection coefficient to generate an up-going wavefield substantially without water bottom multiples.
Ikelle, L. T., et al., in their article, “Kirchhoff scattering series: Insight into the multiple attenuation method”, Geophysics, Vol. 68, No. 1, January-February, 2003, p. 16-28, describe a Kirchhoff scattering series for attenuating surface related multiples in towed streamer data. Ikelle et al. (2003), show how the Kirchhoff series approach with both pressure and vertical velocity measurements is similar to a Born series approach with just pressure measurements.
Tools for surface related multiple suppression have included adaptive subtraction methods based on feedback theory, methods based on the reciprocity theorem, and the inverse scattering derivation methods. All these tools, although based on different theoretical derivations, do not require any knowledge of the underlying subsurface model. In addition, some do also not require knowledge of the source signature. Other methods are also known in the art for suppression of surface related multiples. These methods are commonly known as SRME (Surface Related Multiple Elimination). These are essentially data driven methods, which means that the multiples are predicted from the measured data without knowledge of the subsurface earth model. However, some of these methods require knowledge of the source signature.
However, a common drawback of these methods, as applied to conventional towed streamer data, is the error caused by variation in the sea surface depth and fluctuations in the sea surface reflection coefficient, in addition to streamer feathering and receiver ghosts. These problems are worsened by bad weather conditions, which adversely affect the sea surface. Knowledge of the sea surface and reflection coefficient would allow some correcting of the multiple prediction errors. Thus, a need exists for a method of SRME which effectively attenuates the multiples even in harsh weather conditions.